1. Field of the Invention
The present invention is related to the field of electrical resistivity logging of wellbores. More specifically, the present invention is related to apparatus and methods of measuring electrical resistivity of earth formations from within a wellbore having a steel casing inserted therein.
2. Description of the Related Art
Electrical resistivity measurements of earth formations, made from within a wellbore penetrating the earth formations, can be used to determine the presence of oil and gas in the earth formations. Numerous devices are known in the art for measuring earth formation resistivity. The devices for measuring resistivity known in the art are typically lowered into the wellbore at one end of an armored electrical cable. The devices send signals to equipment at the earth's surface which is electrically connected to the other end of the cable. The signals from the devices correspond to the formation resistivity.
The devices known in the art for measuring resistivity require that the wellbore remain as an "open hole", that is, not have a steel pipe or casing inserted into the wellbore. The devices known in the art for measuring formation resistivity are adversely affected by the presence of the casing in the wellbore since the resistivity of the casing can be smaller than 10.sup.-7 to 10.sup.-10 times the resistivity of the earth formations. The large resistivity contrast between casing and formation disrupts the measurements made by the typical resistivity measuring devices known in the art.
As is understood by those skilled in the art, casings are typically inserted into wellbores in order to maintain the mechanical and hydraulic integrity of the wellbore. It can be desirable from a standpoint of safety and ease of operation to measure resistivity from within a wellbore having casing inserted therein. It is also desirable, for reasons known to those skilled in the art, to be able to re-measure resistivity periodically in wellbores which have been completed and have been producing oil and gas. An apparatus for measuring resistivity in a wellbore having a casing therein is described, for example, in U.S. Pat. No. 5,075,626 issued to Vail.
A particular drawback to the apparatus described in the Vail '626 patent is that this apparatus requires using an electrical logging cable having more than one insulated electrical conductor. The reason that a cable having more than one insulated electrical conductor must be used with the apparatus in the Vail '626 patent is related to the way in which this apparatus measures a property of the formation and casing referred to as the characteristic impedance. A source of electric current is applied between a first electrode in electrical contact with the casing and a return electrode at the earth's surface. Voltage is then measured between an electrode disposed at the earth's surface and a second electrode in electrical contact with the casing and axially spaced apart from the first electrode. The first electrode and second (voltage measurement) electrode must be electrically connected to separate, insulated electrical conductors in order to measure electrical potential difference between the second electrode and the surface electrode while simultaneously energizing the casing and the earth. Although some types of well logging tools use the steel armor on the cable as another electrical conductor to carry electrical power to the tool in the wellbore, in the apparatus of the Vail '626 patent using the steel armor to carry the current from the source instead of using a second insulated conductor would distort the distribution of electrical current which is applied to the casing. The distortion would result from the fact that some of the electrical current would necessarily leak out through the steel armor wires into the wellbore and the casing.
Wellbores which have casings typically are "completed", or placed in a condition where oil and gas can flow to the earth's surface through the casing. Completed wellbores can have substantial fluid pressures within the casing, as is understood by those skilled in the art. In order to insert well logging apparatus into wellbores having substantial fluid pressures within the casing, it is desirable to use electrical well logging cables having small external diameters. Small diameter electrical cables typically include only one insulated electrical conductor. Accordingly, there is a need for an apparatus to measure formation resistivity in a wellbore having casing inserted therein which can operate using an electrical cable having only one insulated electrical conductor.